Transmissions are devices intended for converting rotational speed and torque of a rotational motion source (e.g. a motor) to another device. Three types of transmissions are widely used: mechanical, automatic and continuously variable transmissions (CVT). While geared mechanical and automatic transmissions have a compact size and can transmit a large torque, discretization in the change of transmission ratio requires a torque converter for automatic transmissions that may reduce efficiency, may cause interruptions in torque transmission, and may need a clutch for changing transmission ratios. Furthermore, transmitting torque and power by a rotating continuous belt of variators that can change transmission ratio steplessly and continuously may be difficult.
U.S. Pat. No. 3,608,390 A titled “Variable speed drive” granted to Barrett on Sep. 28, 1971 teaches a variable speed ratio gear system utilizing a conical toothed member made up of an axial stack of gears with varying numbers of teeth, individual gears of the stack consisting of pairs of axially offset pairs, so that the peripheral interface line between adjacent gears has a portion extending generally lengthwise of the conical member.
U.S. Pat. No. 5,653,143 A titled “Automatic mechanical variable ratio transmission” granted to Langevin on Aug. 5, 1997 teaches a variable ratio transmission system for coupling a load to a source of rotational power including a selector shaft having a selector gear axially movable on a fork dolly by an actuator including a lead screw, and a control motor, an inclined cluster shaft having a plurality N of spur gears rotatably supported thereon for being selectively meshed with the selector gear, at least N−1 of the spur gears being variably coupled to the cluster shaft by a one-way clutch for rotation therewith, and a controller for activating the actuator while preventing substantial torque-loading of any of the spur gears unless that spur gear is substantially meshed with the selector gear, the controller comparing a set of operating conditions of the power source with a predetermined operating envelope, and activating the actuator for bringing the set of operating conditions within the operating envelope, the operating conditions including an input operating speed and a set point variable that can be throttle position, torque, power, applied voltage, applied current, modulation duty cycle, or acceleration, the controller being operative for interrupting power from the source for only approximately 0.01 second.
U.S. Pat. No. 6,321,613 B1 titled “Continuously variable transmission” and granted to Avidor on Nov. 27, 2001 teaches a variable ratio transmission for transferring torque between an input port and an output port including two transmission shafts mounted with an angle θ between them. A first of the shafts supports a series of at least two conical gear wheels, of different sizes and having different numbers of gear teeth, in such a manner as to define a predefined range of angular motion through which each conical gear can turn relative to its shaft. The conical gear wheels together define a conical external profile at an angle θ to the first axis. A sliding gear wheel, configured for driving engagement with any one of the conical gear wheels, is engaged so as to slide along, and yet rotate together with, the second transmission shaft. An actuator displaces the sliding gear wheel along the second transmission shaft parallel to the second axis between positions corresponding to selective engagement with each of the conical gears, thereby varying a drive ratio between the first and second transmission shafts.
Russian Federation Patent No. 2,340,815 (also granted as U.S. Pat. No. 6,802,229 B1) titled “Gear drive having continuously variable drive ratio” and granted to Lambert teaches a continuously variable drive ratio transmission including a support, a first shaft rotatably mounted to the support so that shaft can rotate about a first axis, a helical rack supported by the first shaft so that the rack has a plurality of toothed convolutions facing away from and spaced apart along the first shaft, the diameters of said convolutions being such that together they define a conical envelope encircling the first shaft. A second shaft is rotatably mounted to the support so that the second shaft is spaced from and extends parallel to the envelope, and a gear member is mounted to the second shaft for rotation therewith, the gear member having teeth which mesh with those of the rack. The gear member is slidable along the second shaft so that the gear member may be positioned at any location along the rack.
However, such transmissions may have shortcomings: the second shaft gear being at arbitrary positions along the length of the shaft while in rotation may engage with one and then another turn of the rack, resulting in tooth engagement area and, respectively, maximum transmitted torque that may have variable values. i.e. at certain shaft positions loads acting on the edges of the gear teeth of the first shaft and the second shaft may cause a failure. Furthermore, two second shaft gear wheels rotating elastically against each other do not ensure a distinct and definite engagement when changing from one gear transition coil to another. This may result in increased wear, teeth failure or jamming of the transmission.
Therefore, there is a need for a spiral gear transmission.